The detection of trace and weakly absorbing gas-phase species is of importance in scientific, industrial, medical, agricultural and environmental spectroscopic sensing applications. In recent years optical cavity ringdown laser absorption spectroscopy (CRDS) has become a new analytical technique for determination of such trace concentrations. The technique is simple, quick, versatile and an accurate way to acquire weak optical absorption spectra, the method typically able to make optical absorption measurements with sensitivities of the order of 10−7 per cm of sample. General information on CRDS is obtainable for example from U.S. Pat. No. 5,528,040 by Lehmann, “Cavity Ringdown Laser Absorption Spectroscopy: History, Development and Application to Pulsed Molecular Beams” in Chemical Reviews 97 (1997) 25-51 by J. J. Scherer, J. B. Paul, A. O'Keefe, R. J. Saykally, and “Cavity-Ringdown Spectroscopy—An Ultratrace-absorption Measurement Technique”, edited by K. W. Busch and M. A. Busch, ACS Symposium Series (1999) No. 720, ISBN 0-8412-3600-3.
CRDS involves injecting monochromatic light into an optical cavity acting as a high-finesse stable optical resonator (Fabry-Perot optical cavity) formed by two highly reflecting input and output mirrors. A portion of the light incident on one of the mirrors enters the optical cavity and is multiply reflected. When no sample is present in the optical cavity, radiant energy injected into the resonator decreases in time following an exponential decay with a ringdown time τ which is dependent on the reflectivity of the mirrors, their separation and the speed of light in the optical cavity. When a sample is present in the optical cavity, the radiant energy decrease is accelerated at those wavelengths where optical absorption with the sample occurs resulting in a shorter ringdown time τ. An optical absorption spectrum for the sample gas is obtained by placing a detector after the output mirror to detect light emerging from the optical cavity and plotting the energy loss rate which is the reciprocal of the ringdown time (the time profile of light emerging from the optical cavity) versus the wavelength (frequency) of the incident light and comparing this with the optical absorption spectrum of the empty optical cavity. The shape or profile of the resulting plot changes with absorbing species present. For sufficiently weak optical absorption, the ringdown rate increases linearly as optical absorbance or optical absorption coefficient of the sample medium.
CRDS has the advantage that because it is measuring time decay and not amplitude of light it is insensitive to fluctuations in amplitude of light (optical intensity) generated by the light source and is thereby highly sensitive.
As indicated above CRDS is generally capable of detecting optical absorption to a sensitivity of the order of 10−7 per cm of a sample. In certain circumstances, particularly for detecting low concentrations of absorbing species in gases, it would be desirable to achieve greater sensitivities. Detection at greater sensitivities however is not readily attainable with current CRDS systems where problems such as optical feedback, noise present in detection electronics and large optical losses in the systems are present. Further many of the prior art apparatus require the use of large pulsed lasers which are not amenable to portability and are therefore unsuitable in many applications.